ROSCOMMON, a county of Ireland in the province of Con naught, bounded north-east by Leitrim, north-west by Sligo, west by Mayo, west and south by Galway, east by Longford and east and south by Westmeath and Offaly County. The area is 629,633 ac., or about 985 sq.m. Pop. (1926) 83,504. The greater part of the county belongs to the great limestone plain of central Ireland. In the north-east, on the Leitrim border, the Braulieve mountains, consisting of flat-topped ridges, attain an elevation in Cashel mountain of 1,377 ft.; and in the north-west the Curlew mountains, of similar formation, between Roscommon and Sligo, rise abruptly to a height over Boo feet. In the east the Slieve bawn range, formed of sandstone, have a similar elevation. The Shannon forms nearly the whole eastern boundary of the county, and on the west the Suck from Mayo forms for over 5o m. the boundary with Galway till it unites with the Shannon at Shannon Bridge. The other tributaries of the Shannon within the county are the Arigna, the Feorish and the Boyle. The lakes formed by expansions of the Shannon on the borders of Co. Roscommon are Loughs Allen, Boderg, Boffin, Forbes and Ree. Other lakes within the county are Lough Key in the north and Lough Gara (mostly in Co. Sligo) in the north-west. The county town, Roscommon, has a population of 1,83o.
The district was granted by Henry III. to Richard de Burgo, but remained almost wholly in the possession of the native septs. Until the time of Elizabeth, Connaught was included in the two districts of Roscommon and Clare, but in 1579 it was further subdivided by Sir Sydney, and was assigned its present limits. All the old proprietors were dispossessed at the Cromwellian settlement, except the O'Conor family headed by the O'Conor Don. Within the county are the ruins of Crogan, the palace of the kings of Connaught. The principal ancient castles are the stronghold of the M`Dermotts on Castle island, Lough Key, the dismantled castle of the M`Donoughs at Ballinaf ad, and the ex tensive fortress at Roscommon rebuilt by John d'Ufford, justiciary of Ireland in 1268. There are fragments of a round tower at Oran. The abbey of Boyle is in good preservation, and has good Norman arches. The Irish bard Carolan, who died in 1738, is buried by the ruined church of Kilronan, in the extreme north of the county. The bishopric of Elphin was united with Kilmore and Ardagh in 1833, and the former cathedral and episcopal buildings are largely modernized.
The administrative county of Roscommon returns four mem bers to Dail Eireann.